feat: allow any defined response value

[prichey]

The response validator as written fails for valid JSON (e.g. null, "foo", true, etc.). If the validator is going to enforce valid JSON, it should allow all JSON values, not just some. Furthermore, in my opinion this package shouldn't be concerned with validating a user-provided response, perhaps other than asserting that it is defined. Some APIs return invalid JSON - this package should support mocking those, too.

My changes here relax the response validation to merely asserting that it is not undefined. One could argue that undefined should be allowed too, but in this PR I'm only interested in permitting any defined value.

This PR would supersede #201 and close #200

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[prichey]

For posterity: msw-storybook-addon supports this and is regularly maintained